Exposure of fibrinogen receptor on human platelets by proteolytic enzymes.

Human platelets incubated with a-chymotrypsin or pronase did not aggregate with ADP or thrombin and did not agglutinate with ristocetin.On the other hand, they aggregated with fibrinogen even in the absence of platelet-activating compounds.This aggregation was not inhibited by prostaglandin El, sodium azide, or formaldehyde, indicating that it was independent of platelet metabolism.Calcium or magnesium were required for aggregation.Platelets preincubated with a- chymotrypsin or pronase also showed enhanced agglutination with concanavalin A or Lens culinaris lectin.This was inhibited by a-D-glucose, a-D-mannose, and a-D-methylmannoside.These monosaccharides did not inhibit the fibrinogen-induced effect indicating that it differed from that of lectins."'1-Fibrinogen bound specifically to pronase-or chymotrypsin-treated platelets but it did not bind to untreated intact platelets.The chymotrypsin-treated platelets exposed per platelet 1,860 high affinity fibrinogen binding sites (receptors) (Kd = 1.9 X M) and 88,000 low affinity binding sites (Kd = 3.8 X IO-' M). "he mean K , value for fibrinogen calculated on the basis of the rate of aggregation was 1.0 X M (S.E.= 0.3 x lo" M) for chymotrypsintreated platelets.Limited proteolytic degradation of fibrinogen to Fragment X (Stage 1) and Fragment X (Stage 2) resulted in a decrease of platelet aggregation and of affinity to fibrinogen receptor.Fragments Y and D, resulting from the more extensive degradation of fibrinogen molecule, did not have any ability to bind to platelets or to cause their aggregation.It is proposed that the fibrinogen-induced platelet aggregation depends on the occupancy of high affinity fibrinogen receptors.Aggregation of intact platelets by ADP in the presence of fibrinogen and the fibrinogen-induced aggregation of platelets altered by proteolytic enzymes may involve the same receptors on the platelet surface.Fibrinogen is required for ADP-induced platelet aggregation.Suspensions of human washed or gel-filtered platelets do not aggregate with ADP unless fibrinogen is added (1-3).There is also evidence that fibrinogen supports thrombininduced platelet aggregation.Although thrombin aggregates